Of interest in connection with the present invention are the U.S. Pat. Nos. 3,399,659 of Isoda and 4,200,079 of Darlington.
The patent of Isoda relates to a noncompression starting device made integrally with the valve rocker arm chamber of an engine. While starting, a diaphragm having a spindle directly engageable with an exhaust valve rocker arm is urged by a spring acting on one side of the diaphragm to press the exhaust valve to an open or noncompression position. Lubricating oil is introduced on the other side of the diaphragm, and when the engine is cranked to a predetermined speed, the pressure of the oil overcomes the spring biasing force, thus releasing the pressure against the exhaust valve and allowing it to open and close normally.
The patent of Darlington is directed to an engine oil pressure operated system wherein a solenoid valve is electrically energized simultaneously with the starter motor to supply oil pressure from the engine oil pump to a first piston-in-cylinder device, to move a fuel injection pump lever from "stop" to "run" so that fuel supply always depends upon adequate oil pressure. Oil pressure on the piston, opposing the force of a spring, keeps the fuel valve closed after the engine is running. A similar piston-in-cylinder device, which may be used with or without the first one, moves a control arm to change the engine from decompression (exhaust valve open) during starting, to compression when the oil pressure overcomes the force of the spring.
Engine compression release devices according to the present invention are smaller, simpler, and less expensive than those of Isoda or Darlington. They also differ considerably from the prior devices as they do not act on the exhaust valve, but provide an independent path for release of compression pressure.
Darlington's objective is to protect the bearings of large multi-cylinder engines by preventing the engines from firing and/or relieving the cylinder compression pressure until the bearings are well lubricated.
The present invention has a similar beneficial effect by lowering the compression pressures for the initial cranking period, but this is not a primary objective. The primary objective was to minimize size and cost, and overcome some problems associated with the electric cranking motor on a relatively high compression, single cylinder engine.
Small, single cylinder, gasoline engines for lawn and garden service are sometimes equipped with electric starters and sometimes utilize manual starting. Such engines do not have high compression ratios, but they usually employ some means of compression release for cranking. A common type mechanically holds one valve open during cranking until a centrifugal force mechanism comes into play and allows the valve to function normally. On two-cycle engines which have no valves, a very small hole through the piston serves to reduce compression pressure at low cranking speed without having too adverse an effect on performance at normal running speeds.
The present invention was conceived and developed for an electric cranked one-cylinder engine having a high compression ratio. Without compression release, electric cranking is made difficult by two factors. First, the flywheel of the engine cannot be counted on to carry the piston through top dead center on the first compression stroke, since the flywheel may not have achieved sufficient speed. There is no control over exactly where the engine is in its rotational cycle when the cranking motor engages. So the cranking motor must have enough torque and/or a high enough gear ratio to carry through top dead center on its own. The second factor is called "pumping of the Bendix mechanism." After the piston goes over top dead center, it is possible under some conditions that the compression pressure will drive the engine forward instantaneously faster than the cranking motor. This causes the torque to reverse on the overrunning Bendix mechanism, which forces the cranking pinion briefly out of engagement with the ring gear on the engine. The engine has an oil pump. The mechanical compression release schemes were not durable enough for the long life requirements of the engine. The small hole through the piston scheme was not favored because of concern over long-term fouling by carbon deposits and by concern about degrading engine efficiency at low operating speeds. This invention uses the lubricating oil pump to facilitate a simple and long-lived compression release that allows the use of a smaller cranking motor and mitigates the Bendix mechanism pumping problem.